Time division duplex (TDD) applies time-division multiple access (TDMA) principles to two way communications, whereby the time domain is divided into separate repetitive time slots for forward and return signals. Time division duplex has a strong advantage in the case where the asymmetry of the uplink and downlink data speed is variable. As the amount of uplink data increases, more bandwidth (i.e., more time) can be allocated and as the uplink data shrinks the additional time can be used in the other direction. Another advantage is that the uplink and downlink radio paths are likely to be very similar in the case of a fixed or slowly moving system. This means that interference mitigation techniques such as spatial diversity and beam forming work well with TDD systems.
Orthogonal frequency-division multiplexing (OFDM), is a complex modulation technique for transmission based upon the idea of frequency-division multiplexing (FDM) where each frequency sub-channel is modulated with a simpler modulation. A single transmitter transmits on many (typically dozens to thousands) different orthogonal frequencies (i.e., frequencies that are independent with respect to the relative phase relationship between the frequencies). OFDM modulation and demodulation are typically implemented using digital filter banks generally using the Fast Fourier Transform (FFT).
This orthogonality theoretically eliminates all interference between the sub-channels. A number of extra useful benefits, particularly multi-path resistance, arise when the data is coded with some Forward Error Correction (FEC) scheme prior to modulation called channel coding. Moreover, by spreading the transmitted information bits among N subcarriers, the duration of each bit can be longer by a factor of N, and the constraints of timing and multi-path sensitivity are greatly relaxed. However, conventional OFDM suffers from time-variations in the channel. These time variances can be thought of as a carrier frequency offset. This is due to the fact that the OFDM subcarriers are spaced closely in frequency and imperfect frequency synchronization causes a loss in subcarrier orthogonality which severely degrades performance.
TDD/OFDM modulation combines the advantages of both transmission technologies. However, especially in applications involving multiple mobile users communicating with the same fixed base station, it should be understood that it is not a simple matter to maintain accurate frequency synchronization between transmissions from multiple moving transmitters, and thus the same modulation technology need not necessarily be used in both directions. As used in this document, TDD/OFDM merely refers to duplex communications systems in which OFDM is used in at least one direction, and in which bursts of data are being transmitted in only some of the available time slots and received in other time slots.
Thus, in TDD/OFDM wireless communications, particularly in a severe mobile environment with weak transmissions, acquisition and maintenance of the required frequency synchronization at the receiver (or equivalently, compensation for any perceived frequency offset between the respective transmit and receive clocks) with the required degree of accuracy poses a number of technical difficulties, which typically result in not only high processing loads, but also loss of useful communication bandwidth at the start of each transmission burst. Moreover, without fast, accurate and reliable frequency synchronization, the potential modulation efficiencies of OFDM are not obtainable in practice. As a result, conventional TDD/OFDM solutions are not able to adapt to the wide ranges of frequency offsets and changes in signal to interference ratio (SINR) that can be expected in at least some mobile environments without introducing additional processing delays and slower convergence, thus severely compromising system efficiency and communication throughput.